Tribological behaviour ofHVAF-sprayed WC-based coatings: : Role of process variables and binder chemistry

Sammanfattning: Tungsten carbide (WC) based metallic matrix coatings sprayed using high velocity air fuel (HVAF) technique have attracted increasing attention as they show excellent tribological performance in various wear conditions. In the HVAF method, particles’ in-flight temperature and velocity can be influenced by the process variables including nozzle configuration as well as feedstock particle size range. On the other hand, the chemistry of metallic binder can also play a key role in determining both properties and performance of these coatings. In this thesis, characteristics and tribological behaviour of HVAF-sprayed WC-based coatings were investigated employing four different nozzle configurations (4L2, 4L4, 5L2and 5L4), and three different feedstock particle sizes of WC-CoCr feedstock powder (5/20, 5/30 and 15/45 μm). Also, characteristics and performance of coatings processed with four different WC-based feedstocks comprising alternative binders to traditionally used CoCr (namely CoCr, NiMoCrFeCo,FeNiCrMoCu and FeCrAl) were investigated. Characteristics of the coatings were explored by conducting scanning electron microscopy observations, microindentation testing and X-ray diffraction analysis. Performance of the coatings was evaluated by conducting sliding wear, dry jet erosion wear and sand rubber wheel abrasion wear testing. Moreover, material removal mechanisms in the coatings subjected to the above tests were investigated through post wear analysis.X-ray diffraction analysis showed that no considerable phase change compared to the starting feedstock was observed in any of the coatings sprayed by HVAF technique. It was revealed that decrease in powder particle size range can result in an improvement in microstructural characteristics, such as homogeneity and density, as well as hardness of the coatings. Besides, it can lead to a substantial improvement in wear performance of the coatings. It was shown that using various nozzle configurations does not result in any considerable change in characteristics or performance of the HVAF-sprayed WC-CoCr coatings. It was further shown that, by decreasing particle sizes from coarse to medium or fine, a significant difference can be observed. While wear mechanisms for medium and fine feedstock coatings were dominated by ploughing and fracture of individual carbide grains, for the coarse feedstock coatings ploughing and grooving along with the local removal of coating material were observed.Hardness values for all the four coatings with different binder chemistries were in a narrow range of 1100 – 1300 HV0.3. WC-NiMoCrFeCo and WC-FeNiCrMoCu coatings showed better or comparable sliding wear performance compared to WC-CoCr coating (as reference). WC-FeNiCrMoCu and WC-FeCrAl coatings showed comparable performance under erosion conditions while all the three binder alternatives yielded slightly inferior coating performance under abrasion wear, compared to the reference coating. While ploughing was the common wear mechanism in all the four coatings, pitting was noted in coatings with CoCr and FeNiCrMoCu binders in case of sliding wear.